1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) apparatus and a method for controlling the liquid crystal display apparatus.
2. Description of the Related Art
Conventionally, liquid crystal monitors and projectors are known as LCD apparatuses. These LCD apparatuses control the transmissivity of light from a light source by using a liquid crystal panel to display gradations corresponding to an image. To accomplish this control, the LCD apparatuses change a potential difference between a common electrode of the liquid crystal panel and a pixel electrode corresponding to each pixel (i.e., a voltage applied to a liquid crystal between electrodes) to control the polarization direction of the liquid crystal of each pixel of the liquid crystal panel, thus changing the transmissivity of light.
In recent years, some of such LCD apparatuses have employed a new technique for changing the transmissivity of each pixel of the liquid crystal panel. Specifically, the technique maintains a common electrode at a constant voltage, simultaneously inputs a ramp signal having a monotonically changing voltage to each pixel electrode, and turns OFF a switch corresponding to each pixel electrode at a timing according to a gradation displayed by each pixel.
Even this technique enables charging a voltage to each pixel electrode similar to conventional liquid crystal panels, making it possible to change the transmissivity of light of each pixel of the liquid crystal panel as discussed, for example, in Japanese Patent No. 3367808.
To display an image of one line in the horizontal direction, this technique simultaneously inputs a ramp signal having a monotonically increasing voltage to each pixel electrode in one line of the liquid crystal panel, and turns off a switch corresponding to each pixel electrode at a timing according to an image value (gradation to be displayed) of each pixel. In particular, in Japanese Patent No. 3367808, a monotonically increasing ramp signal generated by one reference voltage source 41 for charging pixel voltage is simultaneously applied to each pixel electrode for one line in the horizontal direction (see FIGS. 1 and 2). That is, each pixel electrode in one line shares the monotonically increasing ramp signal generated by the one reference voltage source for charging pixel voltage.
However, when the liquid crystal panel is driven by the method as discussed in Japanese Patent No. 3367808, for example, if there are many pixels having the same gradation (gradation A) in one line as illustrated in FIG. 8A, switches corresponding to many pixel electrodes are simultaneously turned off at a timing when the ramp signal reaches a voltage corresponding to gradation A.
In this case, a load on the one power supply for charging pixel voltage rapidly changes to result in a turbulent ramp signal. In the case illustrated in FIG. 8A, for example, many switches turn off at the same time at a timing corresponding to many gradations A. As a result, the ramp signal corresponding to gradations after gradation A becomes turbulent, as illustrated in FIG. 8B.
If pixel electrode switches corresponding to pixels for displaying gradation B are turned off in a state where the ramp signal corresponding to gradation B (gradation after gradation A) is turbulent, the charge voltage to the pixel electrodes of the pixels for displaying gradation B may become a voltage not corresponding to gradation B.
In this case, the transmissivity of the pixels which should display gradation B of the liquid crystal panel becomes different from the transmissivity of the pixels for displaying gradation B. This signal turbulence is an example, and may be larger for a longer time period. In the example illustrated in FIG. 8A, there has been a problem that a degraded image is presented to a user since the image of gradation B may become brighter or darker than gradation B although the image of gradation A is correctly displayed.